June 12, 2019, Mountain View, CA — The Gemini Planet Imager (GPI), a dedicated planet-finding instrument at the Gemini South telescope in Chile, is concluding a 4-year survey – the GPI Exoplanet Survey (GPIES) – of 531 young, nearby stars searching for giant planets. Analysis of half of the survey data to be published in The Astronomical Journal suggests that giant planets orbiting Sun-like stars, slightly more massive than Jupiter in our solar system, may be rare. GPIES is led by Stanford astronomers and includes an international team.

“We suspect that in our own solar system Jupiter and Saturn sculpted its final architecture that influences the properties of terrestrial planets like Mars and Earth, including basic elements for life like the delivery of water, and the impact rates,” said Franck Marchis, Senior researcher at the SETI Institute and a co-author of the paper. “A planetary system with only terrestrial planets and no giant planets will probably be very different to ours, and this could have consequences on the possibility for the existence of life elsewhere in our galaxy.”

The way GPI searches for exoplanets is different from the methods used in other exoplanet research. Most exoplanets discovered thus far, including those found by NASA’s Kepler spacecraft, are found via indirect methods, such as observing a dimming in the star’s light as an orbiting planet eclipses its parent star (the transit method), or by observing the star’s wobble as the planet’s gravity tugs on the star (the radial-velocity method). These methods have been incredibly successful, but mostly probe the central regions of planetary systems. Those regions outside the orbit of Jupiter, where the giant planets are located in our solar system, are usually out of their reach. GPI, however, endeavors to directly detect planets in this area of space by taking a picture of them alongside their parent stars.

Imaging a planet around another star is a difficult technical challenge reserved to a few instruments, including GPI. Planets are small, faint and very close to their host star. Distinguishing an orbiting planet from its star is like resolving the width of a dime from miles away, and even the brightest planets are ten thousand times fainter, and GPI can see planets a million times fainter than the stars they orbit. Searching for planets this way is also time consuming.

“GPI was designed and built specifically to overcome this challenge, and images from the survey are much more sensitive than those from previous generations of planet-imaging instruments,” said Bruce Macintosh, principal investigator of the project and professor at Stanford University. “The Gemini Observatory gave us time to do a careful, systematic survey. This analysis of the first 300 stars observed by GPIES represents the largest, most sensitive direct imaging survey for giant planets published to date.”

GPI Uncovers a Previously-Hidden Planet and Brown Dwarf

An early success of GPIES was the discovery of 51 Eridani b in December 2014, a planet about two-and-a-half times more massive than Jupiter, and orbiting just beyond the distance Saturn orbits our own Sun. The host star 51 Eridani is just 97 light-years away and is only 26 million years old (nearby and young, by astronomy standards). It had been observed by multiple planet-imaging surveys with a variety of telescopes and instruments, but its planet was not detected until GPI’s superior instrumentation was able to suppress the starlight enough for the planet to be visible.

GPIES also discovered the brown dwarf HR 2562 B, which is at a distance similar to that between the Sun and Uranus and is 30 times more massive than Jupiter. Brown dwarfs are objects that are more massive than planets, but not massive enough to fuse hydrogen into helium like stars. A longstanding question is whether these intermediate-mass objects are born more like stars or planets. Stars form from the top down, by the gravitational collapse of large primordial clouds of gas and dust, while planets are thought—but have not been confirmed—to form from the bottom up, by the assembly of small rocky bodies that then grow into larger ones, a process also termed “core accretion”.

The discoveries made by GPIES, together with its confirmation of an additional five planets and two brown dwarfs that had been detected by earlier generations of instruments, have now shed important new light on questions of formation.

“With six detected planets and three detected brown dwarfs, along with unprecedented sensitivity to planets a few times the mass of Jupiter at orbital distances beyond Saturn’s, our sample can directly answer key questions about wide-separation giant planets and brown dwarfs,” said Eric Nielsen, researcher at Stanford University, previously a postdoctoral fellow at the SETI Institute and lead author of the paper.

“What the GPIES Team’s analysis shows is that the properties of brown dwarfs and giant planets run completely counter to each other,” said Eugene Chiang, professor of astronomy at UC Berkeley and a co-author of the paper. “Whereas more massive brown dwarfs outnumber less massive brown dwarfs, for giant planets the trend is reversed: less massive planets outnumber more massive ones. Moreover, brown dwarfs tend to be found far from their host stars, while giant planets concentrate closer in. These opposing trends point to brown dwarfs forming top-down, and giant planets forming bottom-up.”

More Massive Stars are Over-represented as Hosts of Detected Planets

Out of 300 stars, 123 are more than one-and-a-half times more massive than our Sun. One of the most striking results of the GPI survey is that all hosts of detected planets are among these higher-mass stars, even though it is easier to see a giant planet orbiting a fainter, more Sun-like star.

The relationship between the mass of the star and the giant planet frequency suspected for years has been unambiguously confirmed by this study. The finding also appears to be consistent with the bottom-up formation scenario for planets.

Where are the Jupiters?

One of the greatest surprises that emerged in exoplanet studies has been how different other planetary systems are from our own. While our solar system has small, rocky planets in the inner parts and giant gas planets in the outer parts, we have learned from all exoplanet surveys how intrinsically uncommon giant planets seem to be around Sun-like stars, and how different other solar systems are. Extrapolation of simple models made us believe that GPI would find a dozen giant planets or more, but only saw 6. Putting it all together, giant planets may be present around only a minority of stars like our own. Although GPIES was not sensitive to low-mass planets such as Jupiter, the trends established for higher-mass planets, and in particular, their strong preference to be hosted by stars more massive than the Sun, are clues that one way in which our solar system may be atypical is Jupiter’s presence.

“If this finding is confirmed after analyzing the rest of the survey and more surveys from ground and space-based telescopes to come, it will have an impact on our understanding of the existence of life on terrestrial planets” said Marchis, “and that’s ultimately the raison d’etre of those surveys to understand how planetary system formed and what kind of life could exist elsewhere.”

GPIES observed its 531st, and final, new star in January 2019, and the team is currently following up the remaining candidates to determine which are truly planets and which are distant background stars impersonating giant planets. As this follow-up concludes, the team will move on to publishing the analysis of the entire survey and will begin an upgrade process to make GPI even more sensitive to smaller-mass, closer-in planets as it moves from Chile to Hawaii to begin a new search for planets at the Gemini-North telescope.

This research was based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership. It was funded by the National Science Foundation, the National Aeronautics and Space Administration, the Natural Sciences and Engineering Research Council of Canada, the National Research Council of Canada, Fonds de Recherche du Québec, the Heising-Simons Foundation, Lawrence Livermore National Laboratory, the Center for Exoplanets and Habitable Worlds.

About the SETI Institute

Founded in 1984, the SETI Institute is a non-profit, multi-disciplinary research and education organization whose mission is to explore, understand, and explain the origin and nature of life in the universe and the evolution of intelligence. Our research encompasses the physical and biological sciences and leverages expertise in data analytics, machine learning and advanced signal detection technologies. The SETI Institute is a distinguished research partner for industry, academia and government agencies, including NASA and NSF.

Exiled exoplanet likely kicked out of star’s neighborhood

UC Berkeley Press Release published on December 1, 2015
Written by Robert Sander, Media Relations

A planet discovered last year sitting at an unusually large distance from its star – 16 times farther than Pluto is from the sun – may have been kicked out of its birthplace close to the star in a process similar to what may have happened early in our own solar system’s history.

Images from the Gemini Planet Imager (GPI) in the Chilean Andes and the Hubble Space Telescope show that the star has a lopsided comet belt indicative of a very disturbed solar system, and hinting that the planet interactions that roiled the comets closer to the star might have sent the exoplanet into exile as well.

A wide-angle view of the star HD 106906 taken by the Hubble Space Telescope and a close-up view from the Gemini Planet Imager reveal a dynamically disturbed system of comets, suggesting a link between this and the unusually distant planet (upper right), 11 times the mass of Jupiter. Click image for hi-res versions & caption. Paul Kalas image, UC Berkeley.

The planet may even have its own ring of debris that it dragged along with it.

“We think that the planet itself could have captured material from the comet belt, and that the planet is surrounded by a large dust ring or dust shroud,” said Paul Kalas, an adjunct professor of astronomy at the University of California, Berkeley. “We conducted three tests and found tentative evidence for a dust cloud, but the jury is still out.”

“The measurements we made on the planet suggest it may be dustier than comparison objects, and we are making follow-up observations to check if the planet is really encircled by a disk – an exciting possibility,” added Abhi Rajan, a graduate student at Arizona State University who analyzed the planet images.

Such planets are of interest because in its youth, our own solar system may have had planets that were kicked out of the local neighborhood and are no longer among of the eight planets we see today.

“Is this a picture of our solar system when it was 13 million years old?” asks Kalas. “We know that our own belt of comets, the Kuiper belt, lost a large fraction of its mass as it evolved, but we don’t have a time machine to go back and see how it was decimated. One of the ways, though, is to study these violent episodes of gravitational disturbance around other young stars that kick out many objects, including planets.”

The disturbance could have been caused by a passing star that perturbed the inner planets, or a second massive planet in the system. The GPI team looked for another large planet closer to the star that may have interacted with the exoplanet, but found nothing outside of a Uranus-sized orbit.

Kalas and Rajan will discuss the observations during a Google+ Hangout On Air at 7 a.m. Hawaii time (noon EST) on Dec. 1 during Extreme Solar Systems III, the third in a series of international meetings on exoplanets that this year takes place on the 20th anniversary of the discovery of the first exoplanet in 1995. Viewers without Google+ accounts may participate via YouTube.

A paper about the results, with Kalas as lead author, was published in the The Astrophysical Journal on Nov. 20, 2015.

Young, 13-million-year-old star

The star, HD 106906, is located 300 light years away in the direction of the constellation Crux and is similar to the sun, but much younger: about 13 million years old, compared to our sun’s 4.5 billion years. Planets are thought to form early in a star’s history, however, and in 2014 a team led by Vanessa Bailey at the University of Arizona discovered a planet HD 106906 b around the star weighing a hefty 11 times Jupiter’s mass and located in the star’s distant suburbs, an astounding 650 AU from the star (one AU is the average distance between Earth and the sun, or 93 million miles).

Planets were not thought to form so far from their star and its surrounding protoplanetary disk, so some suggested that the planet formed much like a star, by condensing from its own swirling cloud of gas and dust. The GPI and Hubble discovery of a lopsided comet belt and possible ring around the planet points instead to a normal formation within the debris disk around the star, but a violent episode that forced it into a more distant orbit.

Kalas and a multi-institutional team using GPI first targeted the star in search of other planets in May 2015 and discovered that it was surrounded by a ring of dusty material very close to the size of our own solar system’s Kuiper Belt. The emptiness of the central region – an area about 50 AU in radius, slightly larger than the region occupied by planets in our solar system – indicates that a planetary system has formed there, Kalas said.

He immediately reanalyzed existing images of the star taken earlier by the Hubble Space Telescope and discovered that the ring of dusty material extended much farther away and was extremely lopsided. On the side facing the planet, the dusty material was vertically thin and spanned nearly the huge distance to the known planet, but on the opposite side the dusty material was vertically thick and truncated.

“These discoveries suggest that the entire planetary system has been recently jostled by an unknown perturbation to its current asymmetric state,” he said. The planet is also unusual in that its orbit is possibly tilted 21 degrees away from the plane of the inner planetary system, whereas most planets typically lie close to a common plane.

Kalas and collaborators hypothesized that the planet may have originated from a position closer to the comet belt, and may have captured dusty material that still orbits the planet. To test the hypothesis, they carefully analyzed the GPI and Hubble observations, revealing three properties about the planet consistent with a large dusty ring or shroud surrounding it. However, for each of the three properties, alternate explanations are possible.

The investigators will be pursuing more sensitive observations with the Hubble Space Telescope to determine if HD 106906b is in fact one of the first exoplanets that resembles Saturn and its ring system.

The inner belt of dust around the star has been confirmed by an independent team using the planet-finding instrument SPHERE on the ESO’s Very Large Telescope in Chile. The lopsided nature of the debris disk was not evident, however, until Kalas called up archival images from Hubble’s Advanced Camera for Surveys.

The GPI Exoplanet Survey, operated by a team of astronomers at UC Berkeley and 23 other institutions, is targeting 600 young stars, all less than approximately 100 million years old, to understand how planetary systems evolve over time and what planetary dynamics could shape the final arrangement of planets like we see in our solar system today. GPI operates on the Gemini South telescope and provides extremely high-resolution, high-contrast direct imaging, integral field spectroscopy and polarimetry of exoplanets.

Among Kalas’s coauthors are UC Berkeley graduate student Jason Wang. The research was supported by the National Science Foundation and NASA’s Nexus for Exoplanet System Science (NExSS) research coordination network sponsored by NASA’s Science Mission Directorate.

The Gemini Planet Imager Exoplanet Survey (GPIES) is an ambitious three-year study dedicated to imaging young Jupiters and debris disks around nearby stars using the GPI instrument installed on the Gemini South telescope in Chile. On November 12 2015, at the 47th annual meeting of the AAS’s Division for Planetary Sciences in Washington DC, Franck Marchis, Chair of the Exoplanet Research Group of the SETI Institute and a scientist involved in the project since 2004, will report on the status of the survey, emphasizing some discoveries made in its first year.

Led by Bruce Macintosh from Stanford University, the survey began a year ago and has already been highly successful, with several findings already published in peer-reviewed journals.

“This very large survey is observing 600 young stars to look for two things: giant planets orbiting them and debris disks. In our first year, we have already found what GPI was designed to discover — a young Jupiter in orbit around a nearby star,” said Marchis. This discovery was announced in an article published in Science on Oct. 2, 2015 [http://www.sciencemag.org/content/350/6256/64], with an impressive list of 88 co-authors from 39 institutions located in North and South America. “This is modern astronomy at its best,” said Marchis. “These large projects gather energy and creativity from many groups of researchers at various institutions, enabling them to consider different strategies to improve the on-sky efficiency of the instrument and its scientific output.”

Orbital motion of 51 Eri b detected between two H-band observations taken with the Gemini Planet Imager in December 2014 and September 2015. From this motion, and additional observations of the system, the team of astronomers confirmed that this point of light below the star is indeed a planet orbiting 51 Eri and not a brown dwarf passing along our line of sight. (credit: Christian Marois & the GPIES team)

The survey was officially launched in November 2014. Eight observing runs allowed the study of approximately 160 targets, or a quarter of the sample. Other parts of the survey are more frustrating, though. Due to the incipient El Nino, weather in Chile is worse than expected, with clouds, rain, snow, and atmospheric turbulence too severe even for GPI to fix. Since late June, out of the last 20 nights that team members have spent at the telescope, they’ve only gotten a few hours of good quality data Despite this loss, over which the team of course had no control, they have already published ten peer-reviewed papers in the last year. Two of the findings are described below.

GPI data has revealed that 51 Eri b, the recently discovered Jupiter-like exoplanet around the nearby star 51 Eridani [http://www.gemini.edu/node/12403], indeed has an atmosphere of methane and water, and likely has a mass twice that of Jupiter. The team has continued to observe this planetary system, and observations recorded on Sept. 1, 2015, are most consistent with a planet orbiting 51 Eri and not a brown dwarf passing along our line of sight.

“Thanks to GPI’s incredible precision, we can demonstrate that the odds are vanishingly small that 51 Eri b is actually a brown dwarf that has a chance alignment with this star. In fact it’s five times more likely that I’ll be struck by lightning this year than future data will show this is not a planet orbiting 51 Eri” said Eric Nielsen, a postdoctoral scholar at the SETI Institute and one of the authors of the paper recently accepted for publication in the Astrophysical Journal Letters [http://arxiv.org/abs/1509.07514]. Another author of this study, SETI Research Experience for Undergraduates student Sarah Blunt, analyzed the motion of 51 Eri b and found it to be completely consistent with a planet on an approximately 40-year orbit around its host star.

GPI detection of dust-scattered star light around HD 131835 in H-band linearly polarized intensity. The focal plane mask (filled black circle) was used to block the light from the star (white x). The stronger forward scattering makes the front (NW) side of the disk more apparent. A weaker brightness asymmetry is detected along the major axis with the NE side being brighter than the SW side. By studying resolved images of debris disks, we hope to better understand the giant planet formation and evolution environment (credits: Li-Wei Hung & the GPIES team)

The team has also discovered and imaged disks of dusty debris around several stars. Astronomers believe that these are planetary systems that are still forming their planets. Some have complex structures because they host planets and fragments of the asteroidal and cometary materials that formed those planets. One such system is HD 131835: a massive 15 Myr-old star located 400 light-years from Earth. Using GPI’s high-contrast capability, the team imaged this disk for the first time in near-infrared light in May 2015.

“The disk shows different morphology when observed in different wavelengths. Unlike the extended disk previously imaged in thermal emission, our GPI observations show a disk that has a ring-like structure, indicating that the large grains are distributed differently from the small ones. In addition, we discovered an asymmetry in the disk along its major axis. What causes this disk to be asymmetric is the subject of ongoing investigation, “ said Li-Wei Hung, a graduate student in the UCLA Department of Physics and Astronomy and lead author of the article submitted to the Astrophysical Journal Astrophysical Journal Letters. As asymmetries like the one seen in the system may be due to the gravitational influence of an unseen planet, more detailed observational study could one day confirm its existence.

As the GPIES survey enters in its second year, we are collaborating with the Gemini Observatory to continue to improve the instrument. The Gemini South telescope primary mirror was recently re-coated with silver to improve reflectivity, and the GPI instrument was equipped with a new cooling system to optimize performance.

“Continued collaboration between the Gemini Observatory and the GPIES collaboration has worked really well — we’re learning a lot about how it performs in the field and interacts with the atmosphere, and are working to make GPI an even a better instrument to see even fainter and closer planets,” said Bruce Macintosh, principal investigator of the project and professor at Stanford University.

a) Characterize the surface and atmospheric composition of Galilean satellites and Titan, and monitor the volcanic activity of Io, pinning down the highest temperature of the magma (McEwen et al. 1998; Marchis et al. 2002);

b) Determine size, shape, surface morphology and composition of the 50 largest main-belt asteroids, search for multiplicity, and hence yield information about the bulk density and the formation of this remnant of the solar system formation (Merline, et al. 2002; Marchis et al. 2003);

c) Monitor the atmospheric activity of Uranus and Neptune, focusing especially on the cloud formation and wind profile above the stratospheric haze near the southern pole of Uranus, which is now being exposed to sunlight (Rages et al. 2004). Study of Neptune’s atmosphere yield information about the transport of energy and the source of its mysterious internal source of heat (Pearl & Conrath 1991).

RIGHT: Simulated H-band GPI observations of Jupiter’s small volcanic moon called Io. LEFT: a perfect image of Io as it would appear observed from the ground at an angular resolution of 10 mas (spatial resolution of ~30 km at opposition). Surface albedo features such as pateras, plume deposits and SO2 frost regions are dominant on this image. CENTER: simulated Altair-NIRI observation. Right: simulated GPI image. The ALTAIR-NIRI image was derived using empirical PSFs. The GPI image (right) represents operation in non-coronagraphic mode. The gain in contrast and angular resolution is evident. A faint artificial hot spot located close to the north pole of Io is detectable only by GPI. Detection of the thermal emission in J and H bands is essential to pin down the highest temperature component of the magma and relate directly to the internal structure of the Io (Marchis et al., 2002).

Circumstellar debris disks are the extrasolar analogs of our Zodiacal dust disk (< 3 AU) and the dust complex generated in the Kuiper belt (40-50 AU; ). They are optically thin and gas-poor. Debris disks arise from the collisional erosion of larger solid objects, but may include a contribution from subliming icy bodies as they pass through periastron.

Debris disks offer a secondary pathway for examining planetary systems because planets are responsible for shaping the structure of the disk. For example, concentrations of dust may arise from resonant interactions with planets, and these dust clumps may be seen to rotate around the star over time as the planet proceeds along its orbit. GPI will advance debris disks science in many ways, particularly because it has the ability to analyze the polarization of light reflected from circumstellar grains.

Dual channel polarimetry reveals face-on dust disks and rings. The left frame shows total intensity of a Fomalhaut belt analog, the middle panel shows Stokes Q and the right panel shows Stokes U. The animation begins with a face-on orientation and proceeds to an edge-on orientation.

Polarization measurements serve the dual purpose of suppressing speckle noise (speckles produced by small-angle diffraction are unpolarized) and constraining grain properties. In other words, wavefront errors render the PSF unsuitable for detection of low surface brightness emission from a debris disk. However, light scattered by small grains [x = 2*pi*(grain size / wavelength) = 1, or smaller] in the disk is strongly polarized (> 30%), whereas light diffracted at small angles by wave front errors is only very weakly polarized. As a consequence, the astrophysical signal can be disentangled from the instrumental one. The simulation to the left demonstrates how the dust disk signal is enhanced relative to the noise by using polarimetry.

High-fidelity model GPI data showing a hypothetical debris disk in scattered light. The system consists of two rings viewed at i = 60 ̊. The optical depth of each ring is is similar to HR8799 (Su et al. 2009). The outer ring has a measurable offset caused by an unseen sub-Jovian planet.

Over the last two centuries observations of our own solar system have refined our understanding of planet formation within the context of the nebular hypothesis proposed by Kant (1755) and Laplace (1796). In the last decade a revolution has unfolded as precision radial velocity measurements revealed the first exoplanets, and unveiled an unimagined diversity of planetary systems. The core accretion model for planet formation has been severely challenged by these observations, but remains the dominant paradigm. Direct imaging of exoplanet systems, which is now within our grasp, will pose fundamental new challenges to our understanding of the origin and evolution of planetary systems.

GPI will detect exoplanets in the outer regions (a > 5 AU) of the planetary systems of main sequence stars in the solar neighborhood. By probing large semimajor axis separations that are inaccessible to indirect methods, e.g., Doppler techniques, GPI will reveal the zone where the majority of gas giant planets are expected to reside. GPI surveys will:

Establish directly the occurrence rate of planetary systems;

Provide critical tests of the core accretion model, including a census of regions where gas giants can only form via gravitational instability;

Shed light on the origin of hot Jupiters by finding planets that migrated outwards;

Show whether or not the architecture of our own planetary system, with gas giants located between 5–10 AU is unique.

GPI spatially resolves exoplanets from their parent star and uses an integral field spectrometer to record their spectra at low resolution. About half of GPI detected exoplanets are expected to be cool enough for their atmospheres to contain water clouds. Thus, GPI spectra will provide our first view of the vast terra incognita of cool planetary atmospheres, with temperatures between that of Jupiter and the coolest T dwarfs. GPI may even see the ammonia cloud decks in the coolest exoplanets it detects.

Direct Detection of Exoplanets

In addition to the figure to the left showing a simulated planet detection with GPI, the figures below quantify some of the planet detection statistics with a survey of nearby field stars using GPI.

The figure above shows detectable companion contrast versus angular separation for GPI, with the direct detection of young luminous planets in a hypothetical survey of the field stars within 50 pc.The small dots represent the planet population: those detected by GPI are drawn with a box, those detectable in current Doppler surveys are shown with a circle. The dashed line shows the GPI contrast threshold (5-sigma) for a 1-hour exposure at 1.65 micron. Within 100 lambda/D, speckle noise dominates.

Above, planets detected with GPI are marked with solid filled circles in this plot of planet mass versus semi-major axis. Light dots are planets detected by a hypothetical 8-year astrometric interferometer survey, with a limit of R < 10 mag, and a precision of 30 microarcseconds. Exoplanets detected in the Keck/Lick Doppler survey are shown as stars. This illustrates how GPI explores a complementary phase space to indirect planet searches.

Spectra of Exoplanets

The direct detection of extrasolar planets enables observations of their chemical composition via spectroscopy. The spectrum of a Jupiter reveals the existence of water, ammonia, and methane. The depth and shape of each profile leads to an understanding of the temperature and gravity of planet.

Simulation of a planet detected with GPI. The star is a K spectral type and is 17 pc away. The white object (north) is a background object and the green object below the star is a planet with mass twice that of our Jupiter and with an age of 100 Myr. The white dots near the planet represent the position of the planet relative to the star over 10 years (1 dot/year).

GPI is capable of spectroscopy with R~60 in the near infrared. The solid line shows the spectrum of an extrasolar planet, smoothed to R~40, calculated by Burrows et al. 2003. The dotted line shows the transmission profile through the earth’s atmosphere (higher values represent better transmission). Filled circles represent sample data points from GPI.